Switched reluctance machines have long been known for use in motor drive applications because of their rugged simple construction and ease of control. Only more recently, however, have they actually been implemented in these applications because of developments in power electronics. The typical machine is constructed with a rotor coupled to a shaft which is free to rotate within a stator. The shaft is then drivably coupled to an apparatus such as a pump or other mechanism which requires mechanical energy to operate. The stator of the switched reluctance machine typically has a plurality of diametrically opposed poles which are wound with individual phase windings. The rotor of the machine is constructed from a ferrous material, such as an iron compound, and has a number of salient poles which differs from that of the stator. A unique feature of these machines is that the rotor contains no windings or magnets, and requires no separate excitation. This allows high speed operation without the risk of the rotor flying apart under the centrifugal forces which may be associated with a wound or permanent magnet rotor.
The machine operates by switching current into each of the individual phase windings in a predetermined sequence. This energization of the phase windings creates a magnetic force which attracts a salient pole of the rotor to move into alignment with the opposed poles of the stator, a position which minimizes the reluctance between the stator poles. As the rotor pole moves into alignment, that phase winding is switched off, and the next phase is energized to continue the rotation of the rotor. In this way, a torque is developed to drive the shaft. If it is desired to slow the rotation of the shaft, the phase winding is allowed to remain energized past alignment of the rotor pole, and the magnetic force of attraction produces a negative or braking torque which tries to pull the rotor back into alignment. This results in a braking action, and thus a slowing of the shaft.
During this braking action electrical energy is returned to the bus in the form of a generated current. This occurs as the electromotive force (emf) which is generated in this negative torque region is in the direction to aid current flow. This acts to rapidly increase current flow through the winding. When the switches are turned off, the phase current through the winding will increase for a time, peak, and then decay. This current is forced to flow trough the flyback diodes and back to the dc bus. The net dc current which is returned to the bus is the sum of the currents from all of the phases, and produces a net generating effect. If the rotor were driven from an engine, this mode of operation would produce a net increase of energy flow to the bus, and could sustain the bus voltage under a connected load.
U.S. Pat. No. 5,012,172 describes a control method for use when a switched reluctance machine is operated in this negative torque, or generating mode. The method described therein uses the angular displacement between the rotor and the stator poles as the controlling parameter to regulate the amount of energy returned to the bus. By determining the amount of current required to maintain the bus voltage, the angle at which the switches are turned on is varied. As more current is required, the turn on angle of the switches is advanced. The energy returned to the bus may also be regulated, as disclosed in this patent, by varying the angle at which the switches are turned off.
This method, however, requires the use of a high speed, highly accurate resolver and associated circuitry to determine the exact angular displacement of the rotor pole with respect to the associated stator pole. An error in the monitored switching angle may result in a large error in the amount of current generated, and thus a net increase or a decrease in the bus voltage seen by the utilization equipment. This error may require that the system initiate an overcurrent protection if too great. Although this approach may provide adequate performance, the increased expense on the precision circuitry and the potential for error limits is applicability.
The present invention is directed at overcoming one or more of the above problems.